What holds the wheel together? Astronomers are still not sure (see further below), but have long suspected that a very massive black hole existed at the center of our galaxy, created early in the history of the universe. Their suspicion focused on a compact radio source, also found to emit x-rays, hidden behind dust clouds in the constellation of Sagittarius, the archer.

Now we know more. A large star in that region was found to orbit a dark concentration of mass, estimated at 3.7 million times the mass of our Sun (give or take 1.5 million). The laws of physics have ruled out any explanation but one--that this is indeed an enormous black hole.

Stars near a Black Hole

The public image of a "black hole" is rather menacing, a vortex sucking up anything that comes near, to be swallowed up and never to return. That is not so. The force of gravity near a black hole is indeed tremendous, but like the gravity pull of Sun, Earth or any other object, it obeys the conservation of energy. Any object attracted to a black hole gains velocity and energy, and these may get quite large. However, unless that object happens to be headed directly at the black hole (a relatively small target) it will just swing around and fly away again, like a comet making a pass at the Sun. The velocity acquired in falling towards the black hole also helps it escape.

The star S2

The intense gravity of the black hole prevents any light from escaping it, and it is therefore invisible ("black" indeed!). Its vicinity, however, contains a fairly high density of stars (see here for a picture), including one big star--about 15 times the mass of the Sun and 7 times its radius--which was recently found to go around the center with an orbital period of only 15.2 years (see image above). That star, designated S2 by astronomers, follows an ellipse which at its closest comes within about 124 astronomical units (1 AU=mean Sun-Earth distance) of the center of the galaxy (see illustration). At that time it speeds up to about 5000 km/sec--close to 2% of the velocity of light! Its side facing the black hole is somewhat closer to the black hole than the side facing away, and is therefore pulled more strongly; on a very close approach, such a difference could tear a star apart, but S2 would have to get some 70 times closer before that would happen, at a distance comparable to the orbital radius of Mars.

The observation of this orbit, reported by Rainer Schödel of the Max Planck Institute (Germany) and by his colleagues, is a triumph of Earth-based astronomy. Although S2 is much larger and brighter than the Sun, its visible light is obscured by dust and does not reach us. However, infra-red (IR) light emitted by S2 can penetrate, and a sophisticated IR camera was used, attached to an 8-meter (diameter) telescope of the Southern European Observatory in Chile. The giant mirror telescope overcame the image-blurring twinkling from the atmosphere by using "adaptive optics" with rapidly adjusting mirrors, and attained the incredibly fine resolution about 1/100 of a second of arc (note the scale on the graph above!).

Monster on a Restricted Diet

Far from being a voracious devourer of stars and of interstellar gas, "our own" black hole is rather benign. Gas falling into it causes X-rays to be emitted, but the emission is weak, apart from occasional "flares" thought to come from the capture of comet-sized chunks of matter. A report in "Science" (30 May 2003, page 1356) calls it "The Milky Way's Dark, Starving Pit" and also suggests an explanation. The galactic center is now inside an expanding bubble of gas, apparently created some 10-50,000 years ago when a supernova exploded nearby. The suggestion is that the front of the bubble sweeps away interstellar gas and keeps down the gas density surrounding the black hole.

So is this black hole what holds our galaxy together? Probably not. If it did, then the motion of stars around it would slow down with increasing distance, in accordance with Kepler's third law. The star S2 obeys Kepler's laws, and other stars near the center do so, too. However stars distant from the center do not slow down as much as expected, suggesting their motion is determined, not just by the attraction of the concentrated central mass, but also by some unseen "dark mass" spread out through the galaxy.

The article announcing the discovery (with scientific details): A Star in a 15.2 year orbit around the supermassive black hole at the center of the Milky Way, by R. Schödel et al (22 co-authors), Nature, vol. 419, p. 694-6, 17 October 2002.

A short report in the same issue, describing that work for general readers: Into the heart of darkness by Karl Gebhardt, Nature, vol 419, p. 675-6, 17 October 2002.

Postscript 7.31.2003: Note "The Black Hole at the center of Our Galaxy" by Fulvio Melia, published this year by Princeton University--201 pp, $29.95, reviewed in "Science", 18 July 2003, p. 314. A more detailed discussion of the subject... and it is no more than a coincidence that the titles of the book and of this section are the same!

And by the way...

The Greek legend about a divine mother's milk strewn across the sky is also the source of the word "galaxy," since "gala" in Greek means milk. The ancient Jewish name for the milky way was "river of fire" (nahar di nur).

This section is an extension of our discussion of the Sun. "From Stargazers to Starships" continues with sections dealing with spaceflight and spacecraft, starting with The Principle of the Rocket

However... if you want to extend what you have learned
about the atomic nucleus to find out how nuclear power is
comercially obtained, go next to (S-8) Nuclear Energy and to its recent extension (S-9) Nuclear Weapons